Fix telemetry metrics configuration

[behinddwalls]

Summary

Why?

telemetry.metrics = false did not affect server startup or routing, so the recorder was always installed and /metrics remained available.

What?

Install the Prometheus recorder and authenticated route only when metrics are enabled. Reserve /metrics with an explicit 404 when disabled so the SPA owner route cannot handle it. Add enabled/disabled HTTP coverage to the fast suite.

Test Plan

• ✅ cargo test -p walgit-server --test metrics --test routing_prefix
• ✅ cargo clippy -p walgit-server --all-targets -- -D warnings
• ✅ cargo fmt --all -- --check

Issue

Closes #75

[0bserver07]

Checked this against main and you're right, the recorder gets installed and /metrics is served no matter what telemetry.metrics says. CI hasn't run on this yet, so I ran everything locally with your branch on top of main (build, clippy, unit, e2e and the sims), all green. Small thing: the None check in the handler can't be hit anymore since the route only exists when metrics are on, so one of the two checks could go.

[behinddwalls]

Removed the redundant disabled-state check in 094964f. Route construction now uses the optional Prometheus handle as the source of truth and injects the enabled handle into the handler, so the handler no longer has an impossible None branch. The disabled route still reserves /metrics with a 404.

[addressed by agent]